Method relating to anodic bonding

ABSTRACT

The present invention relates to a method of bonding a first member ( 110, 210, 130, 230, 410, 430, 510, 530, 610 ) to a second silicon member ( 120, 220, 420   a,    420   b,    600 ) through anodic bonding. The method comprises the steps of selectively depositing on said first member bondable sections ( 170   a,    170   b,    270, 470   a,    470   b,    470   c,    570, 620 ) before bringing said first and second members together for anodic bonding.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method of bonding at least a firstmember to a second silicon member through anodic bonding.

BACKGROUND OF THE INVENTION

EP 742 581, for example, relates to a method of making sealed cavitieson silicon wafer surfaces by anodic bonding and with electricallyinsulated conductors through the sealing areas to connect functionaldevices inside the cavities to electrical terminals outside saidcavities. The conductors are provided through the use of doped buriedcrossings in a single crystal silicon substrate, thereby also allowingproduction of different kinds of integrated silicon devices, e.g.sensors.

The technique implies that Borosilicate glass plates are bonded onto asilicon (Si) wafer using so-called anodised bonding. A plate of glass isarranged on a Si wafer under an amount of pressure. The Si wafer and theglass are then heated up to some hundred degrees and a voltage isapplied across the plates (glass and wafer) whereby the glass, whichcontains sodium (Na) ions migrate into the Si wafer, and a hermeticjunction is obtained.

When two Si wafers are to be connected, a similar method as mentionedabove is carried out, however, one of the wafers is coated withsputtered borosilicate glass and an anodised bonding is performed.

Generally, to be able to achieve an anodised bonding the object intendedto be bonded to Si must contain Na-ions, usually through doping a glasswith Soda lime glass. The reason for using borosilicate glass is that itmatches the Si wafer characteristics, specially with respect tocoefficient of expansion.

One major problem related to above mentioned and similar methods is thepossibility of providing a conducting arrangement through the glass orwafer. In above mentioned European Patent No. 742 581, for example, theelectrical connection path is from one of an outside wire bonding areavia a first contact diffusion down to a buried conductor which crossesbelow the sealing area of the cavity, and via a second contact diffusionto a second aluminium interconnection line which establishes connectionsto two piezo-resistors.

SUMMARY OF THE INVENTION

The main object of the present invention is to provide a method ofproviding a hermetical sealing between a first substrate and a siliconsubstrate.

Another object of the present invention is to achieve a hermeticalsealing through selective deposition of bondable surfaces on a firstsubstrate and the silicon substrate. Yet another object of the inventionis to provide electrical connection between the sealed space and theoutside environment through or under the sealing section.

For these reasons, the initially mentioned method comprises the steps ofselectively depositing on said first member at least one bondablesection before bringing said first and second members together foranodic bonding.

Preferably, the first member is a glass wafer, specially a borosilicateglass wafer and said second wafer is a silicon wafer or the first memberis a carrier wafer specially one of glass, ceramics or glass composite,such as LTCC (Low Temperature Cofired Ceramic) and said second wafer isa silicon wafer. The bondable section comprises a paste containing Naions.

Preferably, the selective deposition is provided through screen printingor photo image forming.

Most preferably, depending on the function of the circuitry the bondingis hermetical.

Preferably, said first member comprises of a cover, that said secondsilicon member is a carrier for a functional device and said firstmember bonded to said second member provides a sealing for saidfunctional device. Moreover, a third member is arranged as a carryingmember for supporting said second member.

In one embodiment, electrical connections are arranged out of said coverthrough said bonding sections and/or said third supporting member.

Preferably, said connections through said bonding sections are arrangedon one of said first, second or third members before applying thebonding paste. The bonding sections are provided on said first member.

According to the invention, a method of selectively bonding a firstmember to a second silicon member through anodic bonding is provided,wherein the method comprises the steps of: providing the said first andsecond members, arranging said first member with bonding sections inpredetermined sections, arranging said first and second members in acontacting position, pressing and heating said first and second membersin said contacting position, and applying a voltage to said first andsecond members. Preferably, the second member is a silicon wafercomprising one or more active sections. The first member is a glasswafer provided with frames corresponding to said active sections.

The invention also concerns a sensor comprising a lid, a siliconsubstrate and a carrying substrate, wherein said lid, silicon substrateand carrying substrate are bonded through the method f the invention.

The invention also concerns a biological circuit hermetically connectedto a substrate using the method f the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be further described in anon-limiting way with reference to the accompanying drawings in which:

FIG. 1 schematically illustrates an arrangement produced according tothe teachings of the invention,

FIGS. 2 a, 2 b shows the wafers for anodic bonding process according tothe invention in plan view,

FIG. 3 shows a cross-section along line III—III in FIGS. 2 a and 2 b, ina preassembled form,

FIGS. 4 a, 4 b, 4 c are cross-sections through different schematicembodiments showing wiring according to the invention,

FIG. 5 is a cross-section through yet another embodiment, and

FIG. 6 is a schematic cross-section through a device bonded according toanother aspect of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

According to one preferred embodiment paste, e.g. of thick or thin filmis through, e.g. screen printing or photo image forming, with dopingcontaining Na ions, provided with conductive and non conductive sectionswhich are bondable through anodic bonding. The carrier section may beone of glass, ceramics or glass composite, such as LTCC (Low TemperatureCofired Ceramic).

There are a number of different thick-film pastes with different glassmixtures. It is also possible to produce pastes with sodium or soda-limecontent, both as dielectric and conductive pastes. However, the objectof these is to provide a glass composition which matches the substrateto be printed.

FIG. 1 is a cross section through a device 100, e.g. a sensor accordingto above mentioned sensor of EP 742 581. The device comprises a cover orlid 110, e.g. of borosilicate glass or other glass composition, asemiconducting wafer 120, a substrate 130, preferably a multi layersubstrate including conductors 140 and vias 150 arranged therein andsolder pads 160. The lid 110 is bonded to the Si wafer 120 throughbonding areas 170 a, provided in accordance with the teachings of thepresent invention. The substrate 130 is also bonded to the Si wafer 120through bonding areas 170 b, provided in accordance with the teachingsof the present invention. The bonding areas 170 and 170 b are providedas a paste on the lid 110 and carrier substrate 130, respectively, asclosed frames through screen printing and/or photo image forming or thelike.

The electronic circuitry or functional devices 180 arranged on thesilicon (Si) wafer 120 are connected to the conductors 140, e.g. throughconnections 185 via the Si wafer. It is also possible to arrangeconnections that pass the bonding paste of the connection areas 170 aand/or 170 b, which will be exemplified in the following embodiments.The electronic circuitry 180 is further connected to other circuitsthrough solder pads 160.

In a sensor, filter or similar applications both the lid 110 and thesubstrate 130 can be provided with cavities 190 a and 190 b,respectively.

FIG. 2 a is a plane view of lid wafer 210 of glass on which a number ofsealing frames 270 of a paste material containing Na-ions are printed,e.g. through screen printing. The frames provide a closed space buildingthe cavities 290. On the other side, i.e. FIG. 2 b, functional devices280 are realised on a Si wafer 220, which can be arranged on a carryingsubstrate 230 (FIG. 3), which also is provided with bonding frames orsections 270 b.

FIG. 3 illustrates the moment before the glass wafer 210 of FIG. 2 a isbonded onto the Si wafer 220 of FIG. 2 b. After the bonding processpackaged units are formed, and each unit is cut out later in a suitableway well known for a skilled person.

In FIG. 3, the functional devices 280 may also be countersunk in thesubstrate 220 through micro-machining or the like depending on theapplication and/or the material of the substrate.

The bonding process is performed in a known way, i.e. the Si wafer 220and the lid glass wafer 210 or carrier 230 are combined and exposed to apressure and heat up to a specific level, for example 350° C. (notlimited) and then a voltage, e.g. 800 V (not limited), is appliedthrough the stack comprising the Si wafer and the lid wafer and/or thecarrier.

Here, it is possible to provide different ways of electrical connectionsout of the functional devices arranged inside the sealed area on thesemiconductive material 230: Firstly, according to FIG. 1, i.e. throughthe carrying substrate 130 and secondly through the sealing frame270/270 b. In the embodiment of FIG. 4 a, a substrate 420 a is providedwith conductors 440 a, e.g. through etching or the like. Then the paste470 a (thin film paste) applied onto the glass 410 is pressed on thesubstrate 420 a.

In FIG. 4 b a thick-film paste 470 b is applied through, e.g. screenprinting onto the glass 410. Conductors 440 b having substantially thesame thickness as the paste are arranged through a suitable method onthe substrate 430 b, e.g. alumina.

Clearly, other embodiments are also possible as shown in FIG. 4 c, inwhich a substrate 430 of LTCC is used and into which conductors 440 care immersed. Paste 470 c is applied onto the glass 410 before bonding.

As shown in FIG. 5, it is also possible to countersink the pastes 570into the substrate 530 of LTCC so that the upper surface of the pastecomes into a substantially same level as the upper surface of the LTCC.The glass is denoted with 510.

Additionally, the bonding according to the invention can be used as asealing in further applications. In FIG. 6, for example, a so-calledbiological circuit 600 is connected to a substrate 610. The biologicalcircuit comprises a conduit 601 for transporting fluid or gas. It ispossible to connect and seal the circuit to an external substrate 610of, e.g. LTCC of another circuit likewise provided with a conduit 611using the teachings of the invention, i.e. arranging a ring shaped pastebonding means 620 and anodically bonding the circuit to the substrate orother circuits.

The invention is not limited the shown embodiments but can be varied ina number of ways without departing from the scope of the appended claimsand the arrangement and the method can be implemented in various waysdepending on application, functional units, needs and requirements etc.

1. Method of bonding at least a first member to a second silicon memberthrough anodic bonding, the method comprising the steps of selectivelyapplying a paste on said at least first member at least one bondablesection before bringing said first and second members together foranodic bonding.
 2. The method according to claim 1, wherein said firstmember is a glass wafer and said second wafer is a silicon wafer.
 3. Themethod according to claim 2, wherein glass wafer is a borosilicate glasswafer.
 4. The method according to claim 1, wherein said first member isa carrier wafer chosen from a group comprising glass, ceramics or glasscomposite, and said second wafer is a silicon wafer.
 5. The methodaccording to claim 1, wherein said paste contains sodium (Na) ions. 6.The method according to claim 5, wherein said glass composite is made ofLTCC (Low Temperature Cofired Ceramic).
 7. The method according to claim1, wherein said selective application is provided through one of screenprinting or photo image forming.
 8. The method according to claim 1,wherein said bonding is hermetical.
 9. The method according to claim 1,wherein said first member is arranged as a cover, that said secondsilicon member is arranged as a carrier for a functional device and saidfirst member bonded to said second member provides a sealing for saidfunctional device.
 10. The method according claim 9, wherein a thirdmember is arranged as a carrying member for supporting said secondmember.
 11. The method according claim 10, further comprising arrangingelectrical connections out of said cover through said third supportingmember.
 12. The method according claim 10, further comprising arrangingsaid connections through said bonding sections on one of said first,second or third members before applying the bonding paste.
 13. Themethod according claim 9, further comprising arranging electricalconnections out of said cover through said bonding sections.
 14. Themethod according to claim 1, wherein said bonding sections are providedon said first member.
 15. A sensor comprising a lid, a siliconsubstrate, a carrying substrate and sensor electronics, wherein saidlid, silicon substrate and carrying substrate are bonded using themethod according to claim
 1. 16. A biological circuit on a substratemanufactured according to claim 1 and including a circuit fortransporting a fluid or gas and hermetically connected to saidsubstrate.
 17. A method of selectively bonding a first member to asecond silicon member through anodic bonding, comprising the steps of:arranging said first member with a bonding paste in predeterminedsections, arranging said first and second members in a contactingposition, pressing and heating said first and second members in saidcontacting positions, and applying a voltage to said first and secondmembers.
 18. The method according to claim 17, wherein second member isa silicon wafer comprising one or more active sections.
 19. The methodaccording to claim 18, wherein said first member is a glass waferprovided with frames corresponding to said active sections.